I am writing a book where there is a binary system with a red and blue star. There is planet with the same properties as earth orbiting in a habitable zone around the 2 stars.

The question

How could a binary system be set up so that from the planet during the day the red star lights up the sky but at night only the blue sun can be seen but so that it lights up the night half no more than our moon?

Additional info

-The planet needs to be habitable the whole time

-The parameters of the system can be manipulated in any way you need to accommodate the above description as long as the stars remains blue and red

In short, the red star would be far, far dimmer (even than our own Sol), and would likely be "night", where the blue star would be incredibly bright and would very definitely be "day".

Additionally, the "Goldilocks" zone of such a binary system would be awkward. A blue star is likely to be 4-6x hotter than our own sun, meaning that a habitable planet, for life as we know it, would need to be further away. Additionally, the potentially precarious balance of gravity between two disparate stars alters the Legrangian Points of the star system, and we don't know (so far as I'm aware) what kind of impact that would have on known life.

There are known red-blue binary systems, and some don't hold to my notes above. A great example is Antares, where the red star is a supergiant, and while still very cool for a star is also exceptionally bright and absolutely dwarfs it's blue sister-star.

$\begingroup$Sure, but there's a limit to the distance it can be from the red star to still be a binary system. And for the planet to have any kind of orbit (regular or not) it can't be too far removed from one star while still being close to the other.$\endgroup$
– Jesse WilliamsOct 18 '16 at 21:29

1

$\begingroup$Planets can orbit eider star. There is no such limit.$\endgroup$
– lurchOct 18 '16 at 21:54

$\begingroup$Sometimes it takes thousends of years for them to orbit each other$\endgroup$
– lurchOct 18 '16 at 22:04

It is possible. Systems with two stars can have planets on three kind of orbits: They may be orbiting the red or the blue star or alternatively both together.

The obvious choice is here one that orbits the red star. The blue star could be at any distance. (only not to close) If it is far away it might not really interfere with the habitability, that only depends upon the red star. The blue star would be a small point (but still much brighter than any other star on the night sky) up to a fraction of our the red star. In the later case you might not be able to look directly into it. The blue star might also change during the year. And it could also change during the rotation of the two stars around each other.

However, there are two more possibilities:

The colour of stars depends only on their temperature. That means that the blue star is considerably hotter. I'm here assuming that that are their actual colours, it is possible that they only look like that, but I will discuss that later. The hot blue star might be smaller and therefore be eject less light. In that case the planet could be orbiting both of them together. However in this scenario you may only see the blue star during day and early in the morning or late in the evening, it is always close to the red one. Also for the blue star to be smaller and hotter, it must be considerable older than the red one. (Older stars are hotter.) That means that they cannot have formed together. It is possible, if not very likely, for it to be caught in the system later. However this would make a serious mess with the planets. Some might be ejected from the system, while others might change their orbit. That would make the conditions for live difficult. However if it somehow survives that first phase, everything could stabilise itself. Also the planet might only afterwards become habitable.

The third possibility is that the stars only look blue and red. The sun looks yellow during day but red in the evening. Similar could happen on that planet. Maybe, just maybe, the atmosphere could make them look in the desired way. However I cannot tell you how that might work. This would work with any orbit.

$\begingroup$What you describe would have the blue star move through the sky so it’s on the day side half the orbit. It won’t stay on the night side. So your example does not satisfy the query, so it doesn’t support the summary “it’s possible”. It shows (like my answer) that it’s not possible.$\endgroup$
– JDługoszOct 19 '16 at 7:26

$\begingroup$I in fact did understand the question in such a way that it isn't necessary every night. "Like the moon" he says, which is just part of the nights visible. Also it is possible, if the blue star isn't in the same plain as the planet's orbit. It could be somewhere close to the north pole for hundreds of years and visible during day and night$\endgroup$
– lurchOct 19 '16 at 9:46

Don’t forget that the planet moves. Six months later blue star is in the day sky.

More generally,

if the planet is orbiting both suns, they will be on the same (day) side of the planet.

the night star has to be far away to be on the other side. But the planet orbits the day sun, so half the time they are on the same side anyway.

if the planet were balanced between the two stars, it would be highly unstable (like balancing a pencil on its point), only work for perfectly circular orbit, and doesn’t match the idea of the blue star being far away (so it’s a bright night star not a day sun).

Maybe it’s good enough to have the blue star be in the night sky during the time of the story, moving at a pace much slower than human events. Say, hundreds of years to become the morning star instead.

Also, a blue giant is rather short lived. I like the idea of the star looking blue, perhaps due to a nebula or dust ring that scatters light just like the air does to make the sky blue.

$\begingroup$Hypothetically, if the planet is orbiting one sun, and has the same orbital period as the other sun, would always have one sun in the day and the other at night. Getting the "outer" sun to have the same orbital period as a much-closer planet is an exercise left open to the reader. ;-)$\endgroup$
– AzuaronOct 19 '16 at 1:08

$\begingroup$The other sun is (much) farther away than the planet, right? So how would it have the same orbital period? Orbits are not arbitrary numbers; they follow from matching speed and distance.$\endgroup$
– JDługoszOct 19 '16 at 7:19